Utilization of blast furnace flue dust in ferromanganese production

The amount of raw materials and products used in the world increase as the population rises. Again, because of the increasing population, the requirements for raw material abundance and storage or disposal of waste (or residue) materials are increasing. Iron-steel industry has a serious degree of responsibility regarding this raw materials requirement and disposal of these waste (or residue) materials because steel is being used in many aspects of human life. The demand for steel consumption is increasing since the population is rising and the technology is being improved. In order to fulfill this demand, despite the slowing down in the periods of economic crisis, the global crude steel production reached 1.67 billion tons in 2014 as a result of the increasing trend that occurred during the previous years. Steel sector is among the largest-share sectors in Turkish economy in correlation with the situation in the world. In 2014, approximately 34 million tons of crude steel was produced in Turkey. Two systems are currently being used in iron-steel production. One of these is steel production derived from iron ore in integrated plants while the other is production in electric arc furnaces by using steel scrap. 70 percent of the world's steel production is carried out in integrated plants. Because of this vast production, the input and output items of integrated plants are very important during production. A serious amount of waste (or residue) materials are formed in steel production during the processing of raw materials such as ore, pellet, scrap, coal and limestone in integrated plants. The control of these waste (or residue) materials is essential in the aspects of both energy efficiency and environment. Blast furnace process is the most important production stage in integrated plants. During the production of iron about 2 tons of waste is generated per ton of material obtained. The primary by-products generated during the production of molten iron include blast furnace gas, slag, dust and sludge from top gas cleaning. Gases that are formed as a result of the reactions and the transformations that occur in the blast furnace take the dust outside the furnace with them while they are leaving the furnace. Generally, cleaning the flue gases involves the removal of large particulates by a dry dust collector followed by a wet gas cleaning system for fine particulate removal. These residues are primarily composed of iron, silicon, calcium, magnesium, aluminum as well as carbon in the form of coke breeze. Besides these, wastes may contain significant quantities of heavy metals, which cause serious environmental problems as a result of storage by landfill or spreading on soil. The recovery and reuse of these solid wastes within the same production unit or by different industrial installation are essential for protecting the environment and for conserving metals and mineral resources. Dust and sludge typically constitute 4 to 7% of the produced hot metal with an iron content of 15-40%, carbon content of 25-40% and 7-10% SiO2. These dusts, which contain significant amounts of iron and carbon, must be used properly in order to increase the productivity and prevent their harmful effects to the product quality and the environment. Among these wastes, which are defined as coarse flue dust and sludge related to integrated plants, mixing of coarse flue dust in sintering and pelletizing blends and disposal of sludge are current applications. While recycling of blast furnace flue dust which is formed as a result of production of liquid pig iron is important, manganese is another important material for steel production. Manganese is one of the elements that are extensively abundant in earth's crust. The average concentration of manganese reaches 0.1% of the earth's crust. 90-95% of the produced manganese ore is used in iron-steel production for several aims. Manganese is added to steel as an alloying element in order to increase its strength, toughness and hardness. It also forms sulphides with sulphur in the steel and controls sulphur compounds by this way. The generated manganese sulphide compound prevents the formation of iron sulphide microstructure, which causes hot tearing. Another reason for manganese usage is related the deoxidation duty of manganese in steel. Manganese is the important deoxidizer that is used in deoxidizing operation after aluminium and silicon. Manganese plays an important role in clean steel production because of these reasons. Largely, manganese is used as ferroalloy. In this work, which was carried out because of these reasons, studies and investigations were performed towards the usage of blast furnace flue dust in ferromanganese production. In this work, recyclability of iron and reductiveness of carbon in the composition of blast furnace flue dust, which is formed during production of liquid pig iron in blast furnaces, were investigated on the basis of using blast furnace flue dust in ferromanganese production. In this manner, methods of ferromanganese production were researched and thermodynamic requirements of the subject were investigated as a result of studying previous works that had been completed. Solid-state reduction treatments were applied in argon atmosphere in a tube-type furnace using manganese ore as the source of manganese, mill scale, which is a residue of iron-steel-industry, as the source of iron and coke as the reductant. Optimum reduction conditions that were evaluated by means of theoretical reduction calculations and X rays diffraction analyses are 1200 °C for the experiment temperature, 2 hours for the experiment duration and 200% for the stoichiometric carbon ratio. A reduction degree of 60.97% was achieved in the experiments that were carried out in these conditions. A second stage of solid-state reduction treatments were done in the same conditions by the addition of blast furnace flue dust as the iron source together with manganese ore and coke. The theoretical reduction degree was calculated and X rays diffraction analyses were carried out after the experiments. Analysis results related to the experiments that were done using the blast furnace flue dust were compared with the analysis results of the reduction experiments that were carried out at the first stage. Reducibility of iron oxides in the composition of blast furnace flue dust and reductiveness of carbon were observed. It was understood that the achieved reduction degrees of 68.35% and 67.10% were more than that related to the previous experiments. As a third stage of the experiments, fluxing melting treatments of the metal oxides that had been reduced in solid-state were carried out in an electrical resistance furnace at 1600 °C at the basicity ratio of 1 (CaO/SiO2=1) with the addition of feldspar (CaF2) in the content of 5% of the blend for 1.5 hours. The crucible was broken after the experiments and it was understood that the metal was obtained with the approximate efficiency of 74%. Microstructural investigations, micro hardness measurements and Energy Dispersive Spectrometric analyses were applied to the obtained metallic part. The presence of two different phases was determined as a result of the microstructural investigations. During the micro hardness measurements, the average hardness of one of these phases was measured as 1000 HV and that of the other phase was measured as 790 HV. The presence of phases having hardness values as high as that reveals that the metallic structure is composed of the carbide structures of Mn7C3 and Fe3C. As a result of EDS analyses that were applied to the obtained metal, it was determined that manganese production efficiency was between 78 and 83%. The current study reveals that recycling of iron in the composition of blast furnace can be achieved and carbon can be used as a reductant. It was understood that blast furnace flue dust could be utilized in ferromanganese production in certain conditions in this manner.